Polyolefins produced in a high pressure (HP) process are widely used in demanding polymer applications wherein the polymers must meet high mechanical and/or electrical requirements. For instance in power cable applications, particularly in medium voltage (MV) and especially in high voltage (HV) and extra high voltage (EHV) cable applications the electrical properties of the polymer composition has a significant importance. Furthermore, the electrical properties of importance may differ in different cable applications, as is the case between alternating current (AC) and direct current (DC) cable applications.
A typical power cable comprises a conductor surrounded, at least, by an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor.
Crosslinking of Cables
The polymer material in one or more of said layers is often crosslinked to improve e.g. heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance of the polymer in the layer(s) of the cable. In crosslinking reaction of a polymer interpolymer crosslinks (bridges) are primarily formed. Crosslinking can be effected using e.g. a free radical generating compound. Free radical generating agent is typically incorporated to the layer material prior to the extrusion of the layer(s) on a conductor. After formation of the layered cable, the cable is then subjected to a crosslinking step to initiate the radical formation and thereby crosslinking reaction. Peroxides are very commonly used as free radical generating compounds. The resulting decomposition products of peroxides may include volatile by-products which are often undesired, since e.g. may have a negative influence on the electrical properties of the cable. Therefore the volatile decomposition products such as methane are conventionally reduced to a minimum or removed after crosslinking and cooling step. Such removal step, generally known as a degassing step, is time and energy consuming causing extra costs.
Electrical Conductivity
The DC electrical conductivity is an important material property e.g. for insulating materials for high voltage direct current (HV DC) cables. First of all, the strong temperature and electric field dependence of this property will influence the electric field. The second issue is the fact that heat will be generated inside the insulation by the electric leakage current flowing between the inner and outer semiconductive layers. This leakage current depends on the electric field and the electrical conductivity of the insulation. High conductivity of the insulating material can even lead to thermal runaway under high stress/high temperature conditions. The conductivity must therefore be sufficiently low to avoid thermal runaway.
Accordingly, in HV DC cables, the insulation is heated by the leakage current. For a specific cable design the heating is proportional to the insulation conductivity×(electrical field)2. Thus, if the voltage is increased, far more heat will be generated.
JP2018811A discloses an insulation layer for a DC cable which contains a blend of 2-20 wt % of a high density polyethylene with a low density polyethylene. It is stated that blend provides improved DC breakdown and an impulse property. The blend is mixed with 2-3 wt % of a crosslinking agent. The type and layer structure of the cable has not been specified.
There are high demands to increase the voltage of a power cable, preferably of direct current DC power cable, and thus a continuous need to find alternative polymer compositions with reduced conductivity. Such polymer compositions should preferably also have good mechanical properties required for demanding power cable embodiments.